Much evidence indicates that a cAMP cascade underlies the presynaptic facilitation which is the synaptic mechanism in Aplysia for both the short- and the long-term form of behavioral sensitization of the gill-withdrawal reflex. Moreover, recent work suggests that the same mechanism also underlies classical conditioning of this reflex. We plan to characterize the molecular components of the cAMP pathway. Taking advantage of large identified Aplysia neurons that actually mediate the behavior and which can be microinjected and isolated for biochemical analysis, we would explore the regional distribution of the components of the cAMP pathway within neurons, their heterogeneity of molecular forms, and their relation to cell function. We would characterize the adenylate cyclasc system in sensory neurons, particulary with respect to the control of activity both by the regulatory (G) protein and by Ca++/calmodulin and examine the regulatory subunits of the cAMP-dependent protein kinase which we think is central to all of these forms of presynaptic facilitation. We also will attempt to identify and characterize the protein substrates in sensory neurons whose state of phosphorylation is sensitive to serotonin and which controls transmitter release. Finally, we wish to identify the key components in the neuron that interact with Ca++/calmodulin. Using recombinant DNA techniques, we also plan to determine whether calmodulin exists in Aplysia neurons in a variety of molecular forms each ticketed to regulate specific and different physiological cellular functions. The rationale for this research is its specific relevance to mechanisms underlying synaptic plasticity. In addition, it concerns regulatory pathways that are of general importance to cell biology.